The present invention pertains to detection apparatus and more particularly, to apparatus for detecting the presence of "flat" wheels, i.e., wheels having flat segments, on railroad cars.
A so-called wheel flat results if a wheel of a railroad car or vehicle is so braked or locked that instead of rolling it slides along a rail. When this happens the high friction which develops between the wheel and the rail produces flat segments or portions in the given wheel. It will be understood that the majority of wheel flats appear during the winter because it is at this time that the brake shoes have a tendency to freeze against the wheels which causes the aforesaid sliding and the development of the wheel flat. Other kinds of brake faults can also result in wheel flats, even during mild weather.
In any event and whatever the cause, wheel flats take a significant economic toll in that they require that axles be taken out of service annually. This is because if the wheel flats are left unattended or not repaired, they can cause extensive and serious damage to rails.
Accordingly, it will be appreciated that the aforenoted problems make it important to detect such flat wheels on railroad cars so that they may be taken out of service as soon as practicable for repair; and it is a fundamental object of the present invention to accomplish such detection efficiently and economically.
It will be apparent that wheel flats can be detected either manually or by automatic means, the superior way being by automatic means, because manual detection involves time-consuming wheel inspection and it is difficult to detect a wheel flat because the flat segment or portion is quite often not clearly in the view of the inspector, being on the rail or hidden behind equipment.
For an appreciation of the problems associated with the wheel flats and of an automatic means or apparatus for detecting their presence, reference may be made to a publication by L. M. Ericsson entitled THE NEW WHEEL FLAT DETECTOR JUL400, September, 1974. In that publication, a detailed description is given of such automatic means for detecting wheel flats and the system described is based on the premise that at or above a certain train speed, the wheel suspension system does not permit the wheel flat to drop down and make contact with the rail. More specifically, that system recognizes that at low speed a wheel with a flat will continuously maintain contact with the rail even when the segment with the flat is on the rail. On the other hand, if the speed is higher, such as approximately 30 km/h and higher, the wheel flat will momentarily lose contact with the rail and the time that the wheel loses contact is primarily dependent on the length of the wheel flat and the speed of the train.
Furthermore, in accordance with the prior art system, the speed of the train can be determined by measuring the time it takes for a wheel to pass a fixed measuring distance and this distance may be equal to the circumference of the wheel. The following formula then applies ##EQU1## lp = length of wheel flat L = length of measuring distance .apprxeq. wheel circumference
ts = interruption time PA1 T = reference time = time for the wheel to pass the measuring distance PA1 .DELTA. = factor dependent on load and speed
The formula states that the relation between the length of the wheel flat and the measuring distance is equal to the relation between the interruption time caused by the wheel flat and the passage time over the measuring distance multiplied by a factor .DELTA.. The factor .DELTA. is a function of the weight and speed.
In order to be able to calculate lp, it is necessary to have a fixed measuring distance L and measure the interruption time and the passage time. The measuring distance L is limited by rail mounted wheel detectors for measuring the passage time.
In a simplified version of the prior art system, a voltage applied across the rails from a transmitter is sensed by a receiver and as long as no wheel is inside the measuring distance L the voltage level at the receiver is high. However, when a pair of wheels enters the measuring distance, the voltage is shunted out and consequently the voltage level at the receiver decreases. If one of the passing wheels has a wheel flat and the speed is so high that the wheel loses contact with the rail, the voltage will momentarily increase to a high level. Therefore, it will be appreciated that, in simplified terms, a shunting effect is produced by the presence of passing wheels with an attendant relatively low voltage, but that the voltage will rise for a brief period during the time that a wheel flat is sensed. Not specifically shown in the aforenoted publication is the fact that the high receive level during the passage of a wheel flat may not be a very high level due to the loading effect of adjacent axles.
By measuring the relationship between the time period during which the aforesaid increase in voltage is obtained and the total time during which the transmitter has been shunted out, then the relation between the wheel flat length lp and the measuring distance L in accordance with the previously noted formula can be calculated.
However, it turns out that in practice with conventional bogies or trucks on railroad cars, the distance between the pair of wheels on the truck is between 5.5 feet and 9.3 feet, the wheel circumference being about 9.4 feet since the diameter is approximately 36 inches. The problem here is that both pair of wheels may occupy the measuring section simultaneously when the distance between the pair is equal to, or less than, the wheel circumference. Consequently a wheel flat on one of the wheel pairs would then be shunted out by the other pair of the truck and would never be discovered.
Accordingly, what is done in the prior art system as described in the article cited is that the measuring distance is divided into two equal halves or subsections, each being approximately 4.9 feet in length. Moreover, a transmitter or signal source is provided and a receiver is furnished for each of the subsections. The general layout of such a version can be appreciated by reference to FIG. 11 of the aforenoted article. The limits of the two identical subsections are defined by three inductive rail mounted wheel detectors, such detectors also being used to measure the passage time. The complete operation of such prior art system can be understood by reference to FIG. 13 of the same article.
It will be understood from the aforenoted article that the measuring distance L has to be at least as great as the circumference of the largest diameter wheel that is to be measured by the system. Another governing parameter is that one-half of this measuring distance L should be less than the wheel base of the shortest truck, where wheel base refers to the distance between axles on the same truck.
Whatever the merits of the prior art system referred to, it has been found that certain improvements may be realized in accordance with the present invention.
A primary object of the present invention is to insure that no part of the wheel circumference is missed in the operation of the system; that is to say, that all of the wheel circumference is being sensed to determine whether there is a wheel flat present in any part thereof.
It is another primary object of the present invention to minimize the effect of adjacent wheel axle sets from affecting the sensitivity and performance of the flat wheel detection system.
A further object is to avoid "shorting out" of the signal source or transmitter by a wheel axle or axles.
Another specific object is to provide fixed "shorting means" at each end of the measuring section so as to sharply define the measuring section and thus eliminate the loading effects of wheel axles outside of the measurement section.
In fulfilment of the above stated objects, the present invention provides a scheme in which the track is fed with a high frequency source. As the result of the selection of a high frequency, the rails will exhibit a high impedance at those frequencies, thereby minimizing shunting variations and wheel axle loading effects.
Another primary feature resides in the provision of a transmitter or source which is so connected that the rail feed points are staggered. As a result of this arrangement, the aforenoted object is accomplished of preventing the transmitter or source from being completely shorted by an axle. This feature will be fully explained as the description proceeds.
As above described, the scheme of the present invention normally provides a shorting means fixed at each end of the measuring block or section. However, in the case where certain track circuit equipment might be interfered with, the shorting means may be a series inductance-capacitance circuit so that the shorting means will act as a short only to the frequency involved with the present scheme or system.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.